Problem: Solve for $z$ : $z - 9 = 13$
Add $9$ to both sides: $ \begin{eqnarray} \\ z - 9 &=& 13 \\ \\ {+9} && {+9} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-9 + z} &\hphantom{=}& \hphantom{13} \\ z &=& 13 {+ 9} \\ \end{eqnarray} $ Simplify. $ z = 22$